Computer graphics processing and selective visual display system – Display driving control circuitry – Display power source
Reexamination Certificate
2000-10-30
2004-02-24
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Display driving control circuitry
Display power source
C345S095000, C323S225000, C323S297000
Reexamination Certificate
active
06697060
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a power supply circuit for generating potentials required for driving a liquid crystal, and to a liquid crystal device and an electronic device using same.
BACKGROUND OF ART
FIG. 20
is the configuration of a conventional power supply circuit for generating potentials required for driving a liquid crystal by resistance division. The first to fifth resistors R
1
to R
5
are connected in series across a first potential-supply line
401
supplying a high potential V
0
and a second potential-supply line
402
supplying a low potential V
5
. Potentials V
1
to V
4
between V
0
and V
5
are generated by dividing the potential difference (V
0
-V
5
) between the first and second potential-supply lines by resistors R
1
to R
5
.
These potentials V
0
to V
5
are used as the potentials of common signals COM
0
, COM
1
, COM
2
, and so on applied to common electrodes that are scanning electrodes and of segment signals SEGn applied to segment electrodes that are signal electrodes, as shown in FIG.
16
. In the example shown in
FIG. 20
, potentials V
0
and V
5
become select potentials of common signals, and potentials V
1
and V
4
become non-select potentials of common signals. Potentials V
0
and V
5
become, for example, on-potentials of segment signals, and potentials V
2
and V
3
become, for example, off-potentials of segment signals.
When potentials V
1
to V
4
are generated by resistor division as shown in
FIG. 20
, the current driving capability of a power supply circuit is dependent on the values of the resistors used for dividing voltage. Although a power supply circuit for driving a liquid crystal needs a current driving capability according to the load (liquid crystal) driven by it, the current driving capability of a power supply circuit is limited by the resistors used. In particularly, when the values of the resistors are large and the load of the crystal to be driven is large, the potentials generated by resistor division vary beyond permissible limits. As a result, the liquid crystal display device does not produce a normal display. For a liquid crystal display device to make normal display even in the case where the load to drive the liquid crystal display is large, the current driving capability of a power supply circuit must be increased. This requires the values of the resistors to be decreased. However, decreasing the values of the resistors for resistor increases the power consumption in the power supply circuit.
FIG. 21
is the circuit diagram of another conventional power supply circuit for driving a liquid crystal device, and differs from the power supply circuit of
FIG. 20
in that voltage-follower operational amplifiers
403
to
406
are respectively connected to the output lines of potentials V
1
to V
4
. The voltage-follower operational amplifiers
403
to
406
perform impedance conversion and output of the input potentials V
1
to V
4
.
Although the circuit of
FIG. 21
can decrease the power consumption by the resistors for resistor division, this circuit requires four voltage-follower operational amplifiers
403
to
406
. Furthermore, this operational amplifier has a large power consumption because of requirement of a specific circuit configuration such as differential pair or the like.
An object of the present invention is therefore to provide a power supply circuit for driving a liquid crystal which can decrease the power consumption, and a liquid crystal device and an electronic device using same.
DISCLOSURE OF INVENTION
A first aspect of the present invention provides a power supply circuit for generating potentials used to drive a liquid crystal, the power supply circuit comprising:
first to fourth switches connected in series between a high potential line and a low potential line;
a switch drive circuit which drives the first to fourth switches so that the period of time in which the first and third switches are on and the period of time in which the second and fourth switches are on are alternate; and
a plurality of capacitors of which connection state is switched alternately between series and parallel connections by a switching operation of the switch drive circuit,.
wherein a potential between the second and third switches converges a middle potential between potentials of the high and low potential lines by a switching operation of the switch drive circuit.
According to this aspect of the present invention, the amount of electric charge stored in the plurality of capacitors becomes stabilized because of the switching operation described above. Consequently, the potential between the second and third switches converges the middle potential between the potential difference of the high and low potential lines.
Since no current flows through the circuit when the amount of electric charge stored in the capacitors becomes stabilized, the power consumption can be decreased. In addition, because the potentials become stabilized without being affected by the variation in the capacitances of the plurality of capacitors, an accurate potential can be generated.
When first to third midpoints are midpoints of switch-intervals formed by being divided by the first to fourth switches, the power supply circuit may comprise:
a first capacitor connected between the high potential line and the second midpoint;
a second capacitor connected between the second midpoint and the low potential line; and
a third capacitor connected between the first midpoint and the third midpoint.
By connecting the three capacitors in this manner, the connection of the third capacitor to the first and second capacitors is alternately switched between series and parallel connections by the above-described switching operation.
In this configuration, the first and second capacitors may be replaced by capacitors of a liquid crystal layer formed by supplying potentials of the high and low potential lines and the second midpoint to the liquid crystal layer.
The plurality of capacitors may also be formed of a first capacitor connected between the high potential line and the second midpoint; and a second capacitor connected between the first midpoint and the third midpoint. Further, the plurality of capacitors may also be formed of a first capacitor connected between the second midpoint and the low potential line; and a second capacitor connected between the first midpoint and the third midpoint.
In either configuration, the connection of the first and second capacitors is switched alternately between series connection and parallel connection.
Another aspect of the present invention provides a power supply circuit for generating potentials used to drive a liquid crystal, the power supply circuit comprising: a main power supply circuit generating a potential between potentials of a first potential-supply line and a second potential-supply line; a first sub-power supply circuit generating a potential between potentials of the first potential-supply line and an output line of the main power supply circuit; and a second sub-power supply circuit generating a potential between potentials of the output line of the main power supply circuit and the second potential-supply line. The power supply circuit described above may be used for at least one of the main power supply circuit and the first and second sub-power supply circuits.
By using the power supply circuit described above for all of the main power supply circuit and the first and second sub-power supply circuits, five-level liquid crystal drive potentials V
0
to V
4
used for a ¼ bias driving method can be accurately generated.
To generate liquid crystal drive potentials used for a bias driving method of ¼ or less, for example, six-level potentials V
0
to V
5
, it is preferable to use a resistor division method for the main power supply circuit for generating two-level potentials V
2
and V
3
between the high potential V
0
and the low potential V
5
and use the potentials V
2
and V
3
impedance-convert through impedance-conversion circuits (formed of an operational am
Oliff & Berridg,e PLC
Osorio Ricardo
Seiko Epson Corporation
Shalwala Bipin
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